The present invention relates to force balanced instruments of the closed loop type, and more particularly concerns a force balanced instrument in which position of a sensing mass is capacitatively sensed and the mass is electrostatically forced toward a null position.
In a force balanced sensing instrument, such as an accelerometer for example, it is generally desired that the instrument output signal be proportional to the input condition to be sensed. Therefore, in many types of electrostatic and electromagnetic force balanced sensing instruments special techniques are required to obtain a linear relation between the instrument output and the sensed input. In electrostatic and electromagnetic instruments the forces applied by the instrument forcer are not linearly related to the feedback voltage or current supplied to the forcer. Furthermore, for optimum operation of the instrument itself it is preferred that the feedback force applied by the feedback control network have a linear relation to the sensed input. Thus, special techniques have been employed for obtaining such linearity.
For example, in an electrostatic force balanced accelerometer of the type shown in U.S. Pat. No. 4,679,434, for Integrated Force Balanced Accelerometer, of Robert Stewart, electrostatic forcing in a closed loop system is employed to position and obtain an output from a pendulous inertial mass. The electrostatic forcing system employs a capacitative pickoff electrode on each side of a pendulous member that has been etched from a silicon substrate. The electrodes also apply nominally equal and opposite bias forces to the pendulous member to which is applied a control voltage. In another control arrangement for an accelerometer of this type, a fixed bias voltage and feedback voltage are applied concurrently to pickoff and forcing electrodes on opposite sides of the sensitive mass. The arrangement is such that a net force on the pendulous mass applied by this control system is the difference between the two forces, which is effectively proportional to the feedback voltages, because the fixed bias voltage is a constant.
This system has a number of problems, including the large negative spring effect associated with the required fixed bias electrical fields. Even in the absence of any input acceleration to be sensed, these bias fields are required, and, since the bias field may vary, the instrument may have poor null stability and poor repeatability. In such systems many factors, such as gap variation, aging of components, temperature variations, and the like provide sources of error that may result in spurious output and decreased null stability. Potentially this spurious bias error is a large error. Small variations in electric field are exacerbated by the large negative spring effect associated with voltage control in both parallel and non-parallel motion.
Accordingly, it is an object of the present invention to provide a force balanced instrument that avoids or minimizes above mentioned problems.